SDL2 port

Created by: vlag

Since SDL2 is now RC and its API shouldn't change, we can envisage Solarus to support both 1.2 and 2.0 versions.

The migration guide ( http://wiki.libsdl.org/moin.fcg/MigrationGuide ) well explain all differences, and better, it provides at the final paragraph a header file which will allows to use pretty much the same code for both version.
The "only" exception is that blitting from a hardware surface to another one will result in a no-op under SDL2.